text:
If all fighter jets become stealthy, how will they fight each other in the future? In dogfights?
I came across this question on dogfight-capable fighter jets still being made. If all aircraft become so stealthy, how will they fight each other? Only in dogfights, I presume. Or am I wrong?

Stealthier aircraft causes people to create more sophisticated detection devices (radar, infrared,...), which causes people to create stealthier aircraft, etc...

A related question might be "how do submarines fight each other?"

@CortAmmon - How do submarines fight each other. With very good sonar, quieting, and lots of patience. For sub design, start with Cold War Submarines by Polmar and Moore or Submarine Design by Ulrich Gabler, who designed much of Germany's post-WWII subs. If you have questions, I let me know.

@Hephaestus Aetnaean Thanks for the submarine part. Should we create submarine section in the Area 51?

I'm getting popcorn with all the answers. Stealth fighters will bomb each other's bases to get air superiority...

@user3624251 - Unfortunately, I don't think there'd be enough interest [on SE] to sustain a military.se, much less a sub community. The sub community is pretty small to begin with. (Silent Service, no doubt about it!) If you're still interested in all things naval, we have a thriving and fun group here, including a fair number of service members. Look for Vepr157, our resident Soviet sub expert. And for all your military questions, see r/WarCollege.

This is actually a more pressing issue for submarines than it is for aircraft. https://en.wikipedia.org/wiki/HMS_Vanguard_and_Le_Triomphant_submarine_collision

@J... - You'd be surprised how many times US/USSR subs collided (or almost collided). Early in the Cold War, the USN spent lots of time closely trailing Soviet subs. --- Lots of parallels between anti-sub warfare and 5th gen a/c: lower signature, better signal processing, wideband sensors, short detection ranges, lethal weapons, multistatic sensing (still waiting), hard-kill defenses (upcoming), and exploiting other phenomenology (EO/IR for a/c; non-acoustics for subs).

@HephaestusAetnaean, for a good all-in-one-place picture of submarine warfare some years back, Tom Clancy's The Hunt For Red October can't be beat.  (The Navy tried very hard to classify the manuscript SECRET, even though Clancy could justify every word from open sources.  They didn't want that detailed a picture available in one place.)  For a collection of nonfiction stories of real sub warfare, read Blind Man's Bluff (I forget the author, sorry).

@HephaestusAetnaean, Red Storm Rising was more a complete World War III scenario, using conventional weapons and sited in Europe, rather than strictly a submarine warfare story.  I haven't read SSN, so I can't comment on it.

@JohnR.Strohm - Clancy's Red Storm Rising and SSN also have decent illustrations of ASW (just pick a few chapters from SSN, don't read the entire thing like I did--very repetitive). I've read everything Clancy's written except his last works. --- (btw, Blind Man's Bluff was Sontag. I still have the audiobook on cassette :) ) --- See here for an excellent reading list on naval warfare. I can't recommend enough US Subs Since 1945 and The Last Stand of the Tin Can Sailors.

Then it will be survival of the stealthiest!

They will accidentally crash into each other. ;-)

This isn't primarily opinion based.

Table of contents

Future focii
Short answer - Important: stealth, sensors (radar & IR), data fusion, networking, long-range weapons. Unimportant: dogfighting with guns.
Future counter-detection and counter-measures - Important: RF and IR stealth, lasers, hard-kill defenses
How will stealth fighters fight each other? - including THE MOST IMPORTANT LESSON
Busting myths

Myth: Stealth aircraft can't use radars / radars reveal your location - LPI radar
Myth: IR sensors render stealth obsolete - IR range is short; shorter than radar range against nonstealthy a/c, but greater than radar range against stealthy a/c
Myth: US lags behind Russia/Europe in IR tech - US had less need (better radars); US has very good IR tech
Myth: Stealth makes you invisible to radar  - very hard to see, not invisible
Myth: Stealth is obsolete / a hoax / just marketing - vastly reduced detection range; more efficient jamming; dictate engagements
Myth: WWII/low-freq radars make stealth obsolete / an F-117 was shot down with this one simple trick - Low-freq radars have limited use; F-117 story is untrue.
Myth: most combat is WVR dogfighting - not since the 1960s
Myth: Guns are primary weapons. F-4's were massacred by obsolete MiG-21s in Vietnam because they didn't have guns.

F-4 guns myth - the F-4s did just fine
Welcome to the BVR missile era - where missiles take all the credit, and you usually don't see your enemy

Resources - for modern air combat
Glossary

As aircraft become stealthier, we'll rely more on the following:

Non-radio sensing

IR search and track (IRST)
Optical sensing
(EO/IR are/can be combined into essentially an EO/IR telescope. Future designs will use multiple bands (MWIR, SWIR, color HDTV), have wider fields of view
Mid-future: laser tracking and targeting.
High-powered lasers (100s of kW) for fighters won't debut for decades, but in the meantime they could obtain very high quality bearing, range, speed, and altitude information.

Networking and data fusion

Highly integrated aircraft sensors
Off-board targeting
Eg, a very stealthy sensor platform feeding targeting info to less stealthy (even non-stealthy) launch platforms.

Advanced radars

Multi-band or broadband radars (integrated mid/high-frequency capabilities)
More powerful AESA radars
Better processing
Multi-static arrays to detect side-aspect radar returns.
Side-looking radars.
Again, VLO a/c are not equally stealthy in all directions. They are least detectable from the front aspect.
For instance, the F-22 was to be upgraded with cheek-mounted, side-looking radars, but these were cancelled to reduce cost.

Even better stealth

Short answer
Detection ranges will shrink (against other LO fighters). IR sensors may become the primary means of detection. Fighters may cruise at low subsonic speeds to delay detection.
Radar will still remain the primary sensor for years to come (at least 2 decades, the minimum time for adversaries to field large numbers of LO a/c).
Stealth remains essential.
DATA FUSION and NETWORKING become hugely important; large forces must fight as seamless units. Remote targeting will become more common. Very, very stealthy sensors (eg, F-22 successor) may pass targeting info to less stealthy shooters (eg, F-22, F-35, F-15, missile truck with VLRAAMs)
Future AAMs will feature datalinks and multimode seekers (eg, IR, RF, and laser), improving counter-measure resistance. Many AAMs already have datalinks---shooter guides the missile until the missile's own seeker acquires the target. Advanced propulsion systems debut (multi-pulse motors, ducted-rocket/ramrockets, scramjets), yielding longer range, higher speed, imrpoved kinematics.
Defenses will improve, requiring deeper magazines (smaller weapons and/or larger internal payload bays).
Space, weight, power, and cooling requirements go up. Aircraft grow larger.
Dogfighting will remain rare. Short-range, maneuvering combat will remain unsurvivable with the advent of reliable, high-performance, high Pk, HOBS missiles and helmet-mounted cueing systems. Also, lasers don't miss. (See Myth: guns below.)
5th gen fighters (F-22, F-35) will thoroughly dominate over 4th gen fighters (F-15, Su-35, Typhoon).
Regarding counter-detection and counter-measures
Fighters will better hide their RF and IR signature. See: engine exhaust troughs on F-117, B-2, YF-23, and 6th gen fighter concepts. See: AETD, ADVENT, and F135 upgrades. Tails may disappear.
EW (eg, jamming) will remain important (See: Myth: Stealth a/c can't use radar).
DIRCM (directed IR countermeasures, an IR laser to jam/kill missile IR seekers) will protect fighters and bombers. Already slated for the F-35.
Hard-kill missile defenses will debut (likely small anti-missile missiles in near-term; anti-missile lasers 20+ years hence). Today, we rely primarily on soft-kill defenses, like jamming and decoys.
Answering the question: how will stealth fighters fight each other?
With better sensors and better stealth.
With mid- to long-range missile shots, but mid-range detection (eg, networked, off-board targeting).
With more lethal weapons (faster/short fight time, improved terminal kinematics, less spoofable sensors).
And with better defenses.
In other words, not unimaginably different from how the F-22 sometimes fights today -- a variation on 5th gen combat. Why? [See note.]
Because stealth hasn't changed the most critical component of aerial combat since WWI: SA -- situational awareness -- obtain SA and deny it to your enemy.
THIS IS BY FAR THE MOST IMPORTANT LESSON IN THIS ENTIRE POST. If you learn nothing else, learn this: SA and OODA.
Then do exactly this:

Watch F-18/22/35 pilot and TOPGUN instructor LtCol Chip Berke discuss 5th gen combat.
Go to the resources at the bottom of this post.

Read CSBA's Trends in Air to Air Combat: Implications for Future Air Superiority (or listen to the 1hr presentation) to see how modern air combat is actually fought.
Read The Radar Game to find out why SA (and stealth) is so important.
Read the Gulf War Air Power Survey. Short, concentrated glimpse into modern air wars.

Learn to fly a fighter.

Seriously. Go pick up FSX or DCS and learn. There's a vibrant community to help you out if you get stuck. (You only fly the F-16 in FSX, but it has a dynamic campaign. DCS has far more pilotable a/c. Both are VERY realistic.)
You'll quickly learn that flying the plane takes a lot more effort and is a lot harder than you imagine. You'll spend much of your time flying the plane (or crashing), not actually fighting... which makes sense because it's the same in real life. (The F-14 has two crew for a reason.) You'll quickly appreciate the luxuries of modern conveniences (like auto-trim). "Data-fusion" and "networking" will start to mean something. SA will become the difference between life and respawn.
You'll also learn that dogfighting is very dangerous. You'll rely more on your AMRAAMs than you guns (once you learn to use the radar ;)). You'll resent the Russian IR AAMs, which don't show up on your radar warning receiver. You'll learn the F-16 is not nearly as agile at medium altitude as headlines led you to believe... especially not when loaded with weapons, fuel, and sensors.

To give you a snippet of The Radar Game:

In most victories, the target isn't aware they're being shot at. SA was determinant.
The most successful aces don't engage in gladiatorial 1v1 dogfights -- more like hit-n-run.
Points 1 and 2 apply from WWI to the present day.
Reaction times are measured in seconds, a function of detection range and closing speed. Radar increases your reaction time; stealth decreases your enemy's reaction time.

[Note well: 5th gen platforms already operate differently from their predecessors ("speed is life")].
[Note: it goes w/o saying my prognostications are opinions. Although I'm confident in my predictions -- which are vague but reflect longstanding trends and realistic S&T developments -- I don't pretend certainty.]
[Note: timeframe is near- to mid-future.]

Addressing myths
Myth: Stealth aircraft can't use radars / turning on radars instantly reveals your location
Low-probability of intercept functionality allow you to use your radar without being detected you. Principals of operation include using wider-frequency bandwidth (wideband), frequency hopping, using a frequency-modulated continuous-wave signal, using only the minimum power required for the task, and pulse compression. Wiki has a good summary. AESA radars are particularly suitable because of their thin, pencil-like beams and minimal side-lobes.
Here's a Naval Postgraduate School thesis on the detection and jamming of LPI radars.
This is similar to how LO a/c communicate without being detected. The F-35 is equipped with MADL - multifunction advanced datalink - a stealthy, secure, minimum power, high data rate, line of sight comms system.
Myth: IR sensors render stealth obsolete
Bodies heat up when moving through air.
Vid: F-16 seen through an F-35's EODAS
Vid: F-15E in IR.
IR sensors do not see as far as radar (contrary to other answers here).
Against 4th gen fighters (non-stealthy platforms):

IRST detection range is typically measured in low-dozens of miles.
The F-15's radar ranges out to 100 nmi
The F-22's radar ranges out to 150 nmi
The E-3 Sentry's radar ranges out to 400 nmi

For reference, the F-14's optical (not IR) system could ID and track bombers out to 50 nmi. Perhaps 15-25 nmi for fighters. (Some IR bands are less readily  absorbed by the air than visible light.)
The PIRATE IRST may detect a VLO fighter (with RF and some IR reduction) from these ranges (CSBA):
Altitude  | Mach | Stag temp | Ambient | Contrast | Detection range
==========|======|===========|=========|==========|================
45,000 ft | 1.8  |  85 C     | -56 C   | 141 K    | 150 nmi
45,000 ft | 0.8  | -28 C     | -56 C   |  28 K    |  48 nmi
45,000 ft | 0.5  | -45 C     | -56 C   |  17 K    |

(150 nmi figure assumes no afterburner, ie supercruise.)
While IR is harder than radar to jam at long range, it can't estimate target range as easily.
Myth: US lags behind Russia/Europe in IR tech
Again, IR sensors don't see as far as radar, but they are harder to jam from long range. Historically, NATO fielded better radars and jammers than the Soviets, who therefore couldn't rely as heavily on their radars. So the Russians added IRST capabilities to augment their radars.
Remember also that the US flies dozens of AWACS that usually detect enemy a/c long before even an F-15's radar. So IRST wasn't as critical for US fighters.
The F-22 has a space reservation for IR sensors, but that upgrade was cut to reduce costs.
The F-14 received an EOST relatively early because its main targets were large, Soviet bombers, which are easier to detect.
The US eventually added IRST to late-block F-16's, Super Hornets, and of course the F-35, which fields the most complete IR sensor suite of any fighter. The F-35 has a stealthy, built-in IRST that doubles as a targeting pod. (Demo.) Its DAS also allows pilots to see in a complete sphere (4 pi steradian) around the a/c, even through the floor and headrest (no blindspots); its primary function is missile approach and warning.
The F-35 is an important example here because it'll soon comprise the backbone of NATO aviation for decades. Some 3,400 F-35s are slated for production, more than all F-15s, Super Hornets, Rafales, Gripens, Eurofighters, and Su-27s produced to date combined.
(Btw, if you're skeptical about the F-35 -- kinematics (speed, dogfighting v. F-16), range, stealth, weapons (gun), software, cost -- I'll direct you to these two..comments on kinematics, this compendium, and this community. (To start, a fully loaded F-35 with 5k lbs weapons accelerates like a clean F-16 and points its nose like a Hornet.) If you're still skeptical, I can start another 30,000 character post.)
Myth: Stealth makes you invisible to radar
Not quite.
Stealth vastly reduces your RCS, but does not completely erase it.
VLO a/c are much, much harder to detect and track. Active radar detection range goes with RCS1/4. So a 10x reduction in RCS roughly halves detection range. [See radar equation.]
For example, an F-22 or F-35 has an RCS of 0.0001 m2. A clean [unarmed] F-16 has an X-band RCS of about 2 m2. The F-22/35 will be detected at 12-22x shorter ranges than the F-15/16---effectively under 5-10 miles when using a fighter-class radar. That's almost close enough to see [see note] and certainly within IRST range.
Platform | RCS m^2 | 1/range | Detection range
=========|=========|=========|==================
 F-15    | 25      |  1.0    | 100.
Su-27    | 15      |  1.1    |  88.
 F-16C   |  1.2    |  2.1    |  47.
 F-18    |  1      |  2.2    |  45.
Rafale   |  1      |  2.2    |  45.
Typhoon  |   .5    |  2.6    |  38.
Bird     |   .01   |  7.1    |  14.
Insect   |   .001  | 13      |   8.0
 F-22    |   .0001 | 22      |   4.5
 F-35    |   .0001 | 22      |   4.5

(RCS values are ballpark figures for clean aircraft. External stores greatly increase RCS. A typical F-16 loadout for permissive environments (eg, ISIS, COIN) includes 2x 370 gal fuel tanks, 2x 2000 lb JDAMs (or equivalent), 2x AMRAAMs, 1x Sniper targeting pod. 2x AMRAAMs, 2x AIM-9s, a nav pod (eg LANTIRN), and an ECM pod may be added in more challenging environments. Like this. Not like this. Exception: F-22 and F-35, which carry their stores internally in high threat environments.)
[Note that visual range is theoretically up to 20 miles. But in 99.9% of real life cases it's <5 miles. Often just 2-4 miles.]
Note that once loaded with fuel and weapons, the F-16 and F-15 have much larger RCS values, so will be detected from even further out.
Also, while we no longer use large faceted designs (see: F-117) thanks to modern computing, we still shape our a/c to direct radar energy in certain directions away from the transmitter -- mostly to the sides and above -- in narrow lobes or "spikes."
Illustration (video). Skunkworks engineer discusses designing stealth. Listen closely.
Screencap:

Placeholder: once detect radar transmitter, stay out of detection range. Because detection range is so short, you have a lot more elbow room than non-stealthy fighters.
No, you're not invisible to radar, but operationally, it's much easier to remain undetected.
Placeholder: B-2 spike management. mission planning.

Myth: Stealth is obsolete / stealth is a hoax / stealth is marketing
LO remains relevant (and very useful) for the same reason armor didn't become obsolete when antitank guided missiles were fielded.
LO also makes jamming far more efficient. Jamming basically hides your aircraft's 25 m2 RCS. If your RCS is only 0.0001 m2, it takes far less power to hide yourself, and your jamming will work at far greater range. Primer: how jamming works.
LO technologies drastically reduce detection ranges and warning times. Sensors that work well against stealthy aircraft would work far better against non-stealthy aircraft.
What's the alternative to stealth? No stealth? Make our aircraft easier to detect and track?
All future combat a/c designs must consider signature reduction going forward---at the very least some signature reduction, if not outright VLO designs---camouflaged tanks are always more desirable than this (or this):

Myth: WWII radars / low-frequency radars make stealth obsolete / an F-117 was shot down by an 1960s radar with simple modifications
First, the F-117 story is untrue. The Serbs claimed they made simple modifications to their old Soviet radars, allowing them to easily spot the F-117s. But Zoltán Dani, the Serbian commander who shot down the F-117 in Kosovo, later admitted on national TV that he never modified the radars, merely used the lowest frequency setting. He explained that he fabricated his story for propaganda. Others alleged he did it to take [more] credit for shooting down the F-117.
Also, NATO jamming support was unavailable the night the F-117 was shot down. Dani was aware, so he turned his radars on a third time without relocating, which allowed him to obtain the fateful radar lock.
But more importantly (and accurately) the F-117 was shot down because of lazy mission planning by the US. The F-117 flew the same route night after night at roughly the same hours. So the Serbs moved their radars directly beneath the anticipated flight path. After a number of attempts were made over several nights, they finally got a lock at just 8 mi (13 km) when the bomb bay doors opened. The F-117 doesn't have a radar warning receiver---the pilot never knew he was directly overflying a SAM radar site (doable, but still ill-advised for any stealth fighter). He only ejected because he visually spotted the missile heading for him.
People forget the F-117 is an old design. It entered service in 1983, just five years after the F-16 and nine years after the F-14. Back then, we didn't know how to integrate radars and other apertures into the skin without compromising its low signature.
To be clear, a kill is a kill, and Dani deserves credit. People also forget that the Serbian forces were motivated, competent, and well-trained---measurably more so than Saddam's air defenses. The Serbs performed much better than the Iraqis and learned much from the lessons of Desert Storm.
The F-117 was only shot down once in its entire career, despite regularly flying into some of the most heavily defended airspace in the world, a feat no other platform could perform without massive support (expensive!)...

...or massive losses (save the B-2, F-22, F-35). Stealth alone allowed the F-117 to survive. Even F-15s and Su-35s would have been shot down in droves if we sent them on the same missions alone. (Image source)
Second, low-frequency radars have several drawbacks:

extremely large and power hungry, suitable only for ground-based platforms, not fighters. Some are vehicle-based, but as so large, they're only semi-mobile
require large apertures to function, owing to their long wavelengths
easiest to spoof
easy to detect from long range
detection range is grossly over-advertised
resolution is terrible (on the order of kilometers), making them unsuitable for targeting.
the most straightforward countermeasure to low-freq radars is to make aircraft larger. So VLO bombers (large a/c) are far less detectable by low-freq radars

(Principal: long wavelengths approach the feature size on aircraft, like tails, so shaping is less effective in redirecting radar returns)

all the above make them easy targets for long-range weapons like cruise missiles.
for all the above reasons, they're usually used as early warning radars or to cue higher-frequency radars.

High frequency radars are suitable for targeting, are more jam resistant, and are smaller/lighter. That's why fighters are optimised for stealth at higher frequencies.
Analogy: you can hear a fly buzzing around your room somewhere (detection), but you must see it to swat it (higher resolution sensor for weapons quality track). You can use your ears to cue your eyes, but you'll never hit it with your eyes closed.

(Lo-freq on left, hi-freq on right. SMSgt Mac, elementsofpower.blogspot.com)
Myth: WVR combat / dogfighting is the primary arena of aerial combat
First, WVR combat =/= dogfighting =/= guns.
Second...
Myth: Guns are the primary weapons of aerial combat. F-4's were massacred by obsolete MiG-21s in Vietnam because they didn't have guns.
Common F-4 myth
When Vietnam started, USAF and Navy F-4's claimed just a 2:1 kill/loss ratio. Here commentators often blamed malfunctioning missiles and lack of a gun.
So the USAF added a gun to the F-4, but their K/L remained at 2:1.
In contrast, the Navy began a new program to better train pilots how to use their missiles. They also trained the maintenance crews how to properly handle and maintain their missiles. They did not add a gun.
By war's end, the "sluggish" Navy F-4's claimed a 12:1 K/L ratio against its more maneuverable adversaries like the "agile" MiG-21.
That training program? It became TOPGUN. (Source.)
Welcome to the BVR missile era
The last fighter v. fighter guns kill was in 1989, an Iranian F-4E shooting down an Iraqi Su-22 (CSBA). The F-35's gun is largely for air to ground.
The vast majority of kills over the last 40 years were achieved with missiles:

(Global, fixed-wing v fixed wing. "Other" includes things like CFIT and flying into your own drop tanks. CSBA. See pg 24, pg 36 of 76). Annotation.
In the 1960s, guns still dominated. Most gun kills were achieved by Israelis; they used guns exclusively because no one would sell them missiles.
However, by the 1970s, missiles accounted for more kills than guns.
Even in Vietnam, the AIM-7 Sparrow missile accounted for more kills than any other weapon.

North Vietnam also favored missiles for the balance of its air to air kills, especially as the war progressed.
By Desert Storm, most shots were fired from BVR. Few air to air engagements required any significant maneuvering at all. Only two air to air guns kills were claimed: A-10s vs helos. In another engagement, a helo was destroyed by a laser-guided bomb. In other words, "A2A" bombs killed nearly as many a/c as guns in the Gulf War. Iraq also lost 3 a/c to CFIT.
And yes, we make supremely better missiles today.
Early AAMs were more akin to guided cannon shells: narrow field of view, short range, and low maneuverability; often only chased a target in front of them (tail chase).
Modern AAMs (HOBS, LOAL) have comparatively stupendous ranges: a modern SRAAM (188 lb AIM-9X Sidewinder) has roughly the range of the original AIM-7 Sparrow (a much larger 500 lb MRAAM); and the new AIM-120D AMRAAM (335 lb MRAAM) has nearly the range of the legendary AIM-54 Phoenix (1000 lb LRAAM).
You will not easily outrun a Mach 3 missile pulling 50 g's with a 200° field of view. Why turn 720° when you can shoot almost directly behind you? (video of AIM-9X testing, 2000). (Realistically, some level of maneuverability is required for good shots, but far less than for a guns solution.) More videos, gifs, gfys, and MICA.
Reliability. In ODS: shot 28 AIM-7s at BVR, 22 hit the target or fireball; 10x more reliable than AIM-7s in Vietnam.
Essentially, missiles are now far better dogfighters than fighters.
But the best dogfighting weapon by far is neither gun nor missile: it's a wingman. Fighters don't fly alone. When you dogfight, A) you rapidly bleed speed and altitude, and B) your SA evaporates because you can't afford to focus on anything else except the guy you're turning with. You're effectively a sitting duck and blind, an easy target for anyone outside the fight.
Dogfighting is extremely dangerous. Two modern fighters are liable to both die in a dogfight, ie a kill/loss ratio of 1:1.
I know of no pilot who prefers dogfighting over taking BVR shots.
Dogfighting may not be dead. But it sure is on life support.
Resources

CSBA: Trends in Air-to-Air Combat: Implications for Future Air Superiority --- essential reading
The Radar Game --- Radar and stealth primer. Requisite reading.
RadarTutorial.eu --- radar tutorial. How radars work. Very useful explanations. Explains some of the limitations/necessities of targeting/guidance.
Gulf War Air Power Survey (GWAPS) --- how the Gulf War air campaign was fought (these are some of my notes; the entire report is ~3500 pg).
War with Iraq: Critical Lessons --- General Glosson's book on the war (he orchestrated and designed the campaign) and "explains the deeper strategy and thinking behind the air campaign"
A great selection of books on ODS.
Air Warfare in the Missile Age by Lon Nordeen. Covers every major conflict from 1965 to 2003.
Modern aerial warfare and doctrine.
RCS values on Global Security. Note that the F-35 values were estimates from early in the JSF program, before any production airframes were built. After the F-35 had been built, flown, and tested, officials from the test program (including a pilot) went on record saying the F-35 was actually stealthier than the F-22.
Resources I most frequently recommend

Glossary:

1

AAM - air to air missile
a/c - aircraft
Allied Force - NATO bombing of Yugoslavia, Kosovo War
HOBS - high off-boresight - describing very agile missiles with a wide field of view
LOAL - lock-on after launch
ODS - Operation Desert Storm, part of the Gulf War
RCS - radar cross section
RF - radio freq

2

EO - electro-optical ("telescope")
EOST - " "
IRST - IR search and track ("IR telescope")

3

SR - short range
MR - medium range
LR - long range
VLR - very long range

4

BVR - beyond visual range
WVR - within visual range

5

LO - low observable - ("stealth") a broader term, sometimes interchangeable with VLO
VLO - very low observable - ascribed to only "true" stealth platforms - eg, F-117, B-2, F-22, F-35

(30,000 character limit exceeded)
Placeholder: kinematics still matter
Placeholder: Alleged flaw: Dogfights are more common if US fought USSR instead of easy foes. Why would an adversary with better, longer-ranged sensors and weapons want to get closer where he is also more vulnerable? Related: Combat between peers results in dogfights. Most kills during the 80s resulted from the 8-year Iran-Iraq war between pretty evenly matched opponents. Missiles still dominated. Related: Comparison isn't fair; US had better tech. That's the point. Don't want to fight fair. If high-tech produces fantastic victories, then high-tech sounds like a good idea.

Thanks for taking the time to write up such a long answer; interesting reading.  I wish I'd known there was a glossary before I got to it, though!  Maybe put a little summary and table-of-contents kind of thing at the top, or just a mention of the glossary.  I sometimes do that sort of thing, or put a TL;DR summary at the top of my long answers on stackoverflow.

Excellent work! I agree with @PeterCordes - the glossary could be up front, for convenience. And one minor nit-pic: 3rd paragraph from the end of the F117 shoot-down section, It is completely  - you forgot to finish your sentence. :) Seriously, one of the most thorough answers I've seen in a while!

@PeterCordes, @FreeMan - You're very welcome!!! If you'd like me to add to anything to the glossary, just ask. I forget how many TLAs I use sometimes. table-of-contents Ha! This post actually started off really short... didn't think I'd need one! I'll put it up in sec. I have a bad habit of writing long posts. In my defense, it sounded shorter in my head :) Thanks for the tip, and if you have any other questions at all, please, please let me know.

These arguments have one flaw: all air to air combat involving US fighters in the last few decades was conducted against technologically much inferior opponents, most of whose aircraft were destroyed before they even know what will hit them. However, a fight with an opponent on similar technological levels has high chances to look really different.

@vsz - We fought against less techy opponents. That's the point. But we can still draw lessons from these conflicts. More discussion here. most of whose aircraft were destroyed before they even know what will hit them Welcome to air combat since WWI. This was true of top aces in WW1 and WWII. Still true today. Key is situational awareness. I added a section on SA; lots more info in "The Radar Game." I think you'd be interested.

@vsz - These arguments have one flaw: assume tech superiority Almost none of the arguments assume a tech advantage. In fact, in order to answer the question, the first three sections explicitly assume peer v. peer conflict. The other sections are advantage agnostic; they apply equally to ourselves and our adversaries. The design pressures for peers and near-peers are similar to our own, ie we all want to make roughly similar a/c.

@vsz - However, if you believe these amazing outcomes, these high kill/loss ratios, and this doctrine of air dominance are possible only with a technological advantage... then that seems to prescribe a course of action, doesn't it?

For clarity, I introduced a number of simplifications that aren't strictly accurate. I won't expand on these unless someone asks a question because I've exceeded the 30,000 character limit.

The F-117 doesn't have a radar warning receiver They can pick one up at Crutchfield for 99 bucks. Does the F-117 have a cigarette lighter outlet?

Maybe I'm being a bit pedantic, but the F-117 was shut down over Vojvodina, not Kosovo.

@TomMcW - Cost wasn't the issue. Plenty of other a/c had RWRs. But back then, we didn't know how to integrate radars and other apertures into the skin without compromising its low signature---so the F-117 had no radar!

@AndrejaKo - "In Kosovo" meaning "in the Kosovo War." Thanks for the tip.

(newbie question) I've read that visual-range dogfights are still likely because rules of engagement will often require high certainty target ID (e.g. when you're not in the middle of a full-on war, and REALLY don't want to shoot down wrong targets).  Your answer gives the impression that that would be a bad idea if there's a chance that opponent might have HOBS missiles, because then you'll probably both lose.  Is this accurate, that such ROE would only be viable if the bad guys are known not to have fancy missiles?  (Which might be the case, even if their jets aren't too old, right?)

@PeterCordes - [Pressed for time, will add more later] Short answer: positive ID not an issue anymore. Modern IFF and "other" methods are very good. Example: in ODS, vast majority of a/c in skies were friendly; very few Iraqis; if unknown a/c returns a bad IFF squawk, it's likely a malfunctioning IFF, not an Iraqi. So opportunity for fratricide was high (just like in Vietnam)... but we had no blue-on-blue! Majority of missiles shots were BVR. --- Restrictive ROEs in Vietnam were partly political. Weapon ranges were short anyway.

How about a case like Syria?  What if the US wanted to enforce a no-fly zone over Syria, against Syrian jets?  The Russians will probably keep doing their thing, but we really don't want to make a tense situation worse.  (random link, nothing special).  This seems to me like a fairly plausible situation that would make things complicated.  It's easy to imagine restrictive ROEs being politically desirable even if not technically necessary (even if Syria had newer jets).

I can post this as a separate question, if you want space to properly answer.  Some feedback on what exactly to ask might be helpful, so I can ask a question that will have a good answer (i.e. lines up with something interesting that can be said on the subject).  e.g. How will stealth fighters operate in a complex situation?  Ugh, sounds like a terrible title. :P

@PeterCordes - If you absolutely must see the target, you can use EO/IR, which see much farther than your eyes: https://youtu.be/L2q65qOl1tM?t=42 -- hotel windows clearly visible from 43 miles, seen through the F-35's EOTS. --- btw, in ODS we had AWACS coverage over almost the entire country. We often knew an a/c was hostile because we tracked which airfield it took off from. --- I'll try to get back to you on the rest.

@HephaestusAetnaean I was actually being facetious. I'm full of snarky comments :). But your response does bring up a good question - which I shall post to the site

@fooot - Thanks for the cleanup. And for trimming 100 characters! Sorry for the overwrite, didn't realize you were editing, too.

@HephaestusAetnaean no worries, good work on this answer!

I joined this community just so I could upvote this answer. It is the best I've seen across any SE site. Thank you for your contribution.

@EngineerToast - My pleasure :) If you have any questions, please don't hesitate to ask.

Interesting read! I am new to this topic, so pardon my question: Wouldn't a fight against a technological equal opponent mean that this opponent has stealth measures against the missile lock? And would that not mean that the efficiency of the A2A missiles would be low? (like 1:4 hits or something like that should be possible in the future?) How many missiles does a modern fighter carry?

@Dux - AAMs can't reliably detect LO a/c? Actually, no active-radar missile can detect a target at long range, even a non-LO a/c. Their radars are just too small. So the launch a/c remote controls the AAM toward the target until it gets close enough for its radar to detect and lock onto the target, which is just a few miles (or less) against VLO a/c. This is called a datalink. If the target detects the missile launch (less likely at longer range), he'll try jamming, chaff, or decoys. You can add IR guidance to your AAM help see through radar jamming. (A/c also have IR jammers/decoys.)

@Dux -  How many missiles does a modern fighter carry? Depends on mission and threat. A typical F-16 loadout for bombing in permissive environments (eg, the past 13 years) includes just 2-4 AAMs for self-defense. Like this. F-18s and F-15s can carry up to 10 or 12, but typically far fewer---they need room for fuel tanks. F-22 carries 8 missiles internally; the Block 4 F-35 will carry 6 internally; more externally. Current development aims to double that with smaller missiles. Stealth increases chance your missiles hit, can take better shots.

@HephaestusAetnaean, thanks for the details!
stealth increases chance your missiles hit It might if I use stealth and the opponent doesn't, but a (V)LO target should be hard to lock on both by the a/c and the missile itself? And with increasing technology, the opponent should be able to recognize the missile at larger distances... Unless the missile becomes LO itself, are there technologies for that?

@Dux - So a couple things. 1) It might if I use stealth and the opponent doesn't. Well, if you don't use stealth (but he does), he'll probably shoot first... you won't see him, you won't shoot b/c you're dead... so I'd say your missiles definitely have a lower chance of hitting in that case :P More seriously, with LO, you remain undetected for longer [than no LO], giving you [more] time to position yourself advantageously and set up a shot (eg, behind him; higher speed/altitude to boost your missiles farther)... or at least deny him from doing the same.

@Dux - LO increases your survivability regardless whether your adversary is stealthy or not. (Again, armor analogy.) 2) (V)LO target should be hard to lock on both by the a/c and the missile itself? That's the same situation with current/past 4th gen a/c---obtaining a lock is already difficult due to range, jamming, and decoys. Same issues in Gen5 v Gen5, just shorter ranges. Solutions: better sensors (radar, IR, or both) for a/c and missiles, smarter processing, larger missile salvoes, jamming (to protect your missiles). LO just decreases detection range. Basically.......

@Dux - 3) LO missiles Missiles already have pretty small RCSs.  Modern cruise missiles like JASSM/LRASM also have reduced signatures (eg, shaping, hidden inlets and exhausts). As for LO air to air missiles, the DoD is really tight lipped about current missile programs. Past example: google Have Dash II. Links here 4) MAWS - missile approach and warning systems --- IR: a Mach 3 missile will heat up to 300+ C --- UV: exhaust plume at launch

While there are lots facts, references and interesting things in this answer I don't think any part of it actually tries to simply & clearly answer the headline question. How will they fight if stealth makes them so hard to detect?

@Notts90 - I don't think any part of it actually tries to simply & clearly answer the headline question. See the section "Answering the question."

Using infrared sensors, a fast-moving airplane can be spotted at several 100 km distance if it flies high enough. The friction heating of the fuselage nose and leading edges will stick out against a cold background.
Next, by networking radars and combining the radar data in a sophisticated computer program, stealth aircraft can be tracked reliably even in darkness or clouds. A faceted design will send out a strong return signal over narrowly defined angles, and if you have enough radar receivers, those blips will come fast enough to stitch together the path of the stealth aircraft causing them.
A good part of stealth is propaganda.

Off topic, but is it a feasible idea or a dud to have some sort of inboard cooling system that keeps the nose cold to dampen its IR signature? How hot is the nose typically? I'm thinking order of magnitude what size of cold pack operating on engine bleed or even a consumable cryogenic fluid system would be needed to keep the nose at (say) approx. at the rest of the aircraft temperature.

@curious_cat: Feasible! Like with the radar reflections, you want to point the radiation away from the forward hemisphere. You need to dump heat somehow, but this could be done on the upper rear wing where the IR radiation would not be visible to observers in the target area. At subsonic speed you only need to lower the temperature by a few degrees, but don't underestimate the power consumption. A small pack fed by bleed air will not be sufficient.

@curious_cat - stagnation temperature at Mach 0.85 (a typical cruising speed) is -25 C, modestly above the ambient -56 C at 45,000 ft. This is what an F-16 looks like: https://www.youtube.com/embed/v5VuCsQJy8Y. Typical IR detection ranges are measured in dozens of miles, inside radar detection range (assuming non-stealthy target, no jamming).

@curious_cat - You can dump heat into the fuel, but A) that's difficult to do for the nose because it needs to remain radar transparent, and B) a/c already use the fuel as a heat sink for life support and avionics, so there isn't much marginal heat capacity left (fuel already hot). This is even more important going forward with stealth a/c because LO platforms minimize openings (eg for the ram air intake) to reduce their signature. Future engines will use a third air stream to provide more cooling capacity (and improve efficiency).

@HephaestusAetnaean What about taking along a payload of something like cryogenic N2? Is that a dud because of the large quantity you'd need?

@PeterKämpf Any thoughts on how you'd estimate the pack power needed?

@PeterKämpf If you want to dump the heat to the rear wing that sounds like a heat pump design with some sort of CFC working fluid.

@curious_cat: Not only does N2 evaporate quite easily (you can pour it over your hands, and enough will evaporate to form a protective barrier), but the tank needs isolation making it bigger and heavier. H2O would be better except for the contrails it forms.

@MSalters True. But to cool something down from -25 C you'd need something cooler than that? Unless you can get water to evaporate at that low a Temperature due to the vacuum up there?

@curious_cat - Any cryogenic liquid would be far too heavy/voluminous to carry in any useful quantity---fighters are weight and volume critical. (To be clear, I meant that hot a/c systems heat up the fuel over the course of the mission. Fuel gets hot, doesn't start hot. If it gets too hot, systems shut down.) The most "straightforward" heat management solution is chilling the fuel prior to loading.

@HephaestusAetnaean Makes sense. Are there any extant fighters you know of that pre-chill their fuel prior to loading?

@curious_cat: Pre-chilling helps to pack more fuel into a given volume and is used for record attempts, but not in routine operations. Why don't you post a new question along the line "what can be/has been done for low IR observability?" The comments are inadequate for this topic.

@PeterKämpf Will do. There's some related bits here: http://aviation.stackexchange.com/questions/16876/how-do-stealth-aircraft-reduce-the-heat-signature

@curious_cat - Not that I'm aware of. Only thing coming to mind is painting the F-35's fuel trucks white (the old dark evergreen absorbed a lot of sun). You can read that here: http://www.f-16.net/forum/viewtopic.php?p=298214#p298214. *Just a note: lots of a/c have heat management issues and had to use fuel as a heatsink, this isn't new, just more pronounced for the F-35 b/c it's stealthy (less ram air) and has A LOT of power-hungry avionics. But it's just fine at medium altitude: http://i.imgur.com/zjd4SJy.png

@PeterKämpf - Seeing how you're our resident aerodynamicist and aerospace engineer, and how you've taken considerable time to answer my own questions, it should go without saying I have considerable respect for you. So I'm honestly curious what made you come to this conclusion: A good part of stealth is propaganda.

@HephaestusAetnaean: First of all, congratulations for your answer! It contains a long passage on myths, and some of those are the result of this propaganda. Stealth was sold initially as the cure-all in air warfare (similar to winglets), but is no panacea. It has real costs and can only reduce, not eliminate exposure. It has the biggest benefits against an unprepared adversary, but is sold as if the benefits were absolute. When pilots who participate in maneuvers are routinely sworn to secrecy not to reveal that they located the US stealth aircraft with ease, it makes you think.

@PeterKämpf. Do you have a source who is claiming that the military is placing a gag order on personnel not to reveal that they were able to detect stealth aircraft with relative ease?  I’d be interested to read such accounts.

FIrst, stealth does not make an aircraft invisible- it just makes them harder to detect. Stealth (or correctly, Very Low Observable (VLO)) aircraft can and are detected by radars and other surveillance mechanisms.

Stealth aircraft are usually optimised for low observability in some radar frequency (mostly in Ku- and X- bands). This means that they can be detected by radars in other frequencies at longer distances. Even if the exact location is not known, knowledge of the general location can enable tracking with more powerful radar.
The main thrust of fifth gen aircraft (along with stealth) is sensor fusion and networking of aircraft; basically, if the aircraft can 'talk' to each other and compare their radar returns with that of other aircraft and ground radars, a much better picture of the battlespace emerges, which makes identificaiton of stealth aicraft easier. After all, the VLO aircraft are not equally stealthy from all angles.
Infrared Search and Track (IRST) systems can detect VLO aircraft from much larger distances compared to radars and the US is just now warming up to the idea- Both Russian and European aircraft have been using IRST for quite some time.
Another point is that stealth comes at a cost- for targeting enemy aircraft, you have to switch on you radar. The inverse square law of radar strength means that you'll be detected much before the radar returns from the opponent will be detectable by you. One way to think about the fight between two VLO aircraft is that it's like fight between two gunslingers with flashlights in a dark room. Here, the first one to switch on the flashlight (radar) is at a distinct disadvantage.
One tradeoff in VLO design is that that weapons are carried internally. During weapons release, the radar signature of the aircraft spikes, which can aid in detection.
Finally, the VLO characteristics are important only in BVR area. Once the dogfight turns into WVR, stealth aircraft has no distinct advantage over any other aircraft (see the F-35 Vs F-16 case).

Ad Both Russian and European aircraft have been using IRST for quite some time: Can you give examples of such aircraft?

MiG 29 and Su 27 and all their derivatives, e.g. see here [https://en.wikipedia.org/wiki/Mikoyan_MiG-29#/media/File:German_MIG-29_Nose.jpg], as for the europeans: Gripen, Eurofighter and Rafale.

F-35 Vs F-16  - The F-35 has numerous advantages over the F-16, even WVR: higher instantaneous turn rate (19 vs 14 deg/s) and similar sustained turn rate (8-9 deg/s) at 20k ft; higher sustained AOA (50 v 26 deg; F-35 tested to 110 deg), better acceleration (50,000 lb fully loaded F-35 with full 5,000 lbs of internal weapons accelerates like a clean F-16) ; helmet mounted missile cueing; and 360 IR and "HUD".
Those F-35 turn rates are while carrying 2x to 3x as much fuel as the F-16 and 5,000 lbs of internal weapons, while the F-16 carries just 1,700 lbs (6 missiles).

F-35 vs F-16 - If you're referring to the F-35 "dogfight" report, I HIGHLY suggest reading the actual report, not the highly selectively quoted David Axe hit piece. It wasn't a dogfight; it was a test flight to tweak the control laws. The F-16 was used as a visual aid to maneuver against.

Appendix
I exceeded the 30,000 character limit in my above answer. Here are some additional figures and quotes.
CSBA
On dogfighting and WVR:

Hitting an alerted opponent was really hard, according to aces in WWI and WWII. ALL viewed maneuvering combat as high risk, low payoff.
Ericc Hartmann, Gerhard Barkhorn (300+ kills each on the eastern front in WWII) developed "ambush tactics": position of advantage, swoop in, shoot, exit, reassess, repeat.
Richard Bong and Thomas McGuire developed similar tactics ("boom and zoom")
Post-Vietnam Red Baron air reports: 80% of kills resulted from attacking unalerted victims. Superior SA was key.

Desert Storm

ROE - contact is declared hostile if...

AWACS declares contact hostile
2 sources of ID (eg, visual, Mode 4 IFF, non-cooperative target ID available on F-15Cs)

Coalition killed ~33 Iraqi a/c

27/33 - AWACS provided targeting info.
16/33 - BVR kills
17/33 - WVR engagements, only 5 included any significant ACM. Long range detection afforded pilots time to position themselves advantageously for shots without pulling significant g'.

BVR engagements

Longest detection range was 42 nmi (basically the F-15C's radar limit). Longest missile shot was 16 nmi---10 nmi was average---AIM-7 missile limitation. The longer ranged AMRAAMs weren't available yet. Crews often just waited to get into range.
Shot 28 AIM-7s in BVR engagements. 22 hit the target or fireball.... new AIM-7s were nearly 10x more reliable than 1960s AIM-7s in Vietnam.

Speed

US aircrews never exceeded 650 knots.

Misc

Longest BVR missile kill: 62 nmi in mid 1980s by Iranian F-14 with Phoenix missiles.

To answer the original question: The same as conventional aircraft do.
Beyond visual range fights - guided missile technology developed from the 1950s onwards provided fighter designers with a weapon which could track and destroy an enemy aircraft at ranges where they could not be visually identified.  In theory these weapons seemed like a simple, ideal solution.  In practice, however these weapons have had mediocre, if not downright dismal results due to both their technological complexity and the complexity of their associated mission systems carried by the launching aircraft. Missile kinetic energy to maneuver with an evading target was also limited by the motor and the requirement of the launching aircraft to continuously track the target and relay mid course guidance corrections to the missile until either impact or terminal guidance by the missile's own onboard radar system.  The post above notes how the air to air kills have shifted over from guns to radar guided missiles but it incorrectly assess the reliability of these weapons.  For instance it states that during the Gulf War / Operation Desert Storm that 28 AIM-7s were launched by F-14, F-15 and F-18 fighters during the conflict with 24 kills as a result.  This is false; 88 AIM-7s were launched with 24 kills as a result giving a AIM-7 Probability of a Kill (Pk) at 27% as opposed to the originally suggested 85%.  In addition, while there are a few known BVR kills in that conflict most of the kills on record using the BVR SARH AIM-7 were either at undocumented ranges or were within visual range shots.  Virtually all of the documented kills using BVR missiles required the cooperation of an target aircraft flown by an ignorant or incompetent pilot.
Air battles post-ODS have been difficult to analyze as we have had so few battles to provide an adequate sample size to authoritatively review; what we have seen of BVR weapons in these few fights is not inspiring.  For instance on Jan 5, 1999 during Operation Southern Watch, a pair of Iraqi MiG-25s violated the southern no-fly zone over Iraq and tracked a pair of F-15s patrolling this airspace.  The Eagle crews responded with a BVR attack using 3 AIM-7 and one AIM-120A missiles.  All four missiles missed the MiGs.  A pair of Navy F-14s joined the fight and launched two AIM-54 Phownix missiles at the Foxbats; these too, also missed and the MiGs escaped unharmed.
And if our radar guided BVR weapons have lackluster reputations, Russian and Chinese types are even worse.
In 1999 as well four Eritrean flown MiG-29s engaged four Ethiopean Su-27s flown by Russian mercenary pilots resulting a total victory for the Flankers; all four MiG-29s were shot down.  Of interest in this duel were the weapons used.  The Sukhois launched 3 R-73 (AA-11) IR guided dogfighting missiles at close range, scoring three kills; Some 24 AA-7 radar guided missiles were exchanged by both sides at BVR ranges before one AA-7 managed to damage one of the MiGs, causing it to exit the fight (the MiG crashed while attempting to land so the kill is credited).
Dogfighting - The factors of aerial warfare make air combat maneuvering an inevitability.  It has been with us since Manfred Von Richtofen dueled over the skies of Europe during WWI, it will be with us when the last of the F-22s bake in the Tucson sun.  Stealth technology makes this all the more inevitable as potential threat nations around the world are developing their own stealth aircraft which will be more difficult for our forces to detect.  This reduces detection and tracking ranges and increases the chances that opposing aircraft will stumble into each other by accident resulting in a visual merge.  Missiles, and in particular the latest generation short range high off-boresight LOAL IR guided weapons couples with helmet mounted cueing systems have changed the nature of close range aerial duels, but the gun does reamain an effective and necessary weapon to posses at extremely close ranges or other situations where such a guided AAM cannot be employed.

The post above I wrote that post. I agree with you top to bottom. Rereading my post (now 6 mo later), I realize some folks may leave with the wrong take-away. I don't have the character allowance (or time) to revise it now, but I want folks to know I agree with what you say here (and elsewhere).

I want to clarify one thing: This (In ODS: shot 28 AIM-7s at BVR) refers to only the 28 AIM-7s shot at BVR, not all AIM-7s shot. Of those 28, 22 hit the target or FIREBALL, so fewer than 22 a/c were actually hit.

The factors of aerial warfare make air combat maneuvering an inevitability Just to highlight this, 7 F-35s were "lost" in Red Flag 17-1, all at WVR. (They also got 145 a/a kills)

No.  If you read the source you will find that 88 AIM-7s were fired during the entirety of ODS with only 28 confirmed hits.  I'm also skeptical of the recent Red Flag exercise and what the F-35's supporters are claiming as 'kills'.  I suspect these figures are being rigged or inflated to produce something which does not parallel the reality of that program.

17 Feb 2017 HASC testimony excerpts on F-35 kill ratios. Full recording if you have time: starts at 49:00. I heard up to 24 red air, 3 or 4 regens.

Great - and these are simulated kills and there is no definitive information on how these kills were obtained.  The reality of war seems to say otherwise in terms of weapons employed vs what actually hit their intended targets.

An important addition consideration is realizing is many aircraft historically are destroyed on the ground.
In Operation Desert Storm, of the ~140 Iraqi aircraft destroyed in the way, 105 of them were destroyed on the ground. The largest portion of Iraqi craft destroyed were not brought down in air-to-air combat.
Even in WWII, large numbers of planes were destroyed on the ground.
It is not overly unrealistic to expect that in the future as planes continue to get stealthier that more will be destroyed on ground as compared with in air.


text:
Why does rudder cause roll?
In the few rudder incidents with the B737, rudder-freeze caused the airplane to roll. From what I know, the rudder causes yaw while the aileron causes roll. Why in the case of these accidents/incidents would the rudder cause a roll?

This has already been asked on physics.SE although not here, strangely.

Think about the Dutch roll that can be the result of a yaw perturbation. Yaw causes slip, slip causes roll, roll is reversed by the yaw stability. The yaw damper minimizes this sequence. Good article on stability and how effects are interlocked: Flightlab Ground School - 4. Lateral/Directional Stability. Related topic, page 4-13.

There are many higher-order effects. For example, yaw causes a slight increase in airspeed over the outer wing of the turn and a decrease in airspeed over the inner wing. As a result, the outer wing will generate more lift than the other, introducing a rolling motion.
Yaw also causes side-slip which triggers several effects. Wings that are swept backwards, as on the B737, contribute to the roll towards the direction of the turn, because the outer wing gets effectively longer (visual explanation) and produces more lift. Further contributing to this roll is the outer wing having a higher angle of attack due to the dihedral effect (visual explanation).
Similar effects apply to other control surfaces. It's not true that rudder only causes yaw and ailerons only cause roll. Those are simplifications, even though aircraft are usually designed for these simplifications to hold fairly well.
There is also a direct effect. The image below didn't show the center of mass; I drew in a guess as a yellow circle. It's clearly below the rudder, which is true for most aircraft. This means applying rudder introduces a rolling torque, roughly equal to the sideways component of the force on the control surface, multiplied by the distance between rudder and the longitudinal axis. The magnitude of this product is visualised as the area of the red rectangle.

The two effects - direct and yaw-induced appear to oppose each other, right?

@BrianDrummond: On many aircraft, in many flight conditions: yes, the direct effect is in the direction of the turn and the net torque of indirect effects is in the opposite direction, but that's not true in general. E.g., some less orthodox aircraft designs have a tail fin below the longitudinal axis. This tail would cause a plane to roll left when yawing right and vice versa. Also, many fighter aircraft have anhedral wings, reversing the dihedral effect to reduce stability. My guess is such exceptions are rare, but I don't have the numbers to back that up.

" It's not true that rudder only causes yaw and ailerons only cause roll." that's why we call them coordinated turns

Agreeing with @Marcks Thomas.  If you look at the Predator drone, you'll note that it uses an "inverted vee" tail, where the control surfaces are part elevator, part rudder.  If you deflect both surfaces to port, it will do two things: a) cause you to yaw to port, because the deflection is aft of the center of gravity, and b) cause you to roll to port because the deflection is BELOW the center of gravity.  In that regard, the Predator typically doesn't need to use the ailerons to achieve a proper, coordinated turn.

The reasons can be split in two categories: Direct and indirect.
Direct reasons are rolling moments which are created directly due to the rudder deflection and the side force on the vertical tail:

Offset position of the vertical tail: Since the rudder is above the longitudinal axis of inertia, a side force will also cause a rolling moment. This is the main reason for the anhedral of the Lockheed F-104.
Once the yawing motion starts, the forward-moving wing experiences a slight increase in dynamic pressure, so lift is increased on the forward moving wing and vice versa. This effect is inverse to flight speed and small in general.

Indirect reasons are caused by the resulting sideslip angle once the aircraft starts to yaw:

Dihedral: Once the rudder-caused yaw produces a sideslip angle, the relative difference in angle of attack between both wings creates a rolling moment. This is proportional to the dihedral angle in flight, so you need to add wing bending to a static view of the aircraft to get an idea of the effective dihedral angle.
Wing position: On a high wing, the changed flow around the fuselage in sideslip will create a difference in angle of attack at the root of the wing. This effect needs to be considered when selecting the proper dihedral angle; therefore high-wing aircraft have less dihedral than low-wing aircraft.
Wing sweep: The reduced sweep angle of the windward wing will increase its lift curve slope, and the reverse happens on the opposite wing. This creates a strong rolling moment which is proportional to the sweep angle.

Once the sideslip angle increases, the rudder-induced side force will decrease until the aircraft reaches the trim point where the sideslip angle fully cancels the rudder deflection. But the aircraft is still yawing, so it will exceed the trim position and now the side force will act in the opposite direction. Proportionally to the side force, the tail-induced rolling moment will also change due to the buildup in sideslip.

I kept looking for your name, I knew this one would attract you! Love that it's a simple comprehensive list of causes. Wish I could upvote more than once.

@egid: I am currently at CES Unveiled and have little time for edits. Typed it this morning in the hotel room when I wasn't satisfied with the other answers.

After stopping by at our physics friends, it's probably because of the swept wings (causing a lift differential) of the B737:

[...] if you introduce a yaw to the aircraft, one wing will extend out more directly into the wind-stream, while the other wing will be even more swept. This effectively makes one wing longer, and the other wing shorter. [...] The longer wing will generate more lift, and the shorter one will generate less lift. And since there is unequal lift around the roll axis, the airplane will roll, and continue to roll.

The whole answer:
Physics: What causes an aircraft to roll when rudder is applied
Note: Regarding the comments, this effect even occurs without swept wings. Swept wings only amplify the effect.

Note, that A320 mechanical law (the last fallback when all electricity fails) only has elevator (stabilizer actually) and rudder control and relies on this effect for making turns. The aircraft was demonstrated fully controllable in this mode.

Although it does have an effect, even an aircraft without swept wings experiences a roll force when rudder is applied.

This is an incomplete answer, possibly misleadingly so. The lift differential is the key here, not the wing sweep.

Agreed with the other comments. The wing sweep plays a part, but it's only a part and this happens in aircraft that don't have swept wings. For example, I've been in a Cherokee that was flown around the pattern with no aileron input.

When an aircraft yaws, one wing travels faster than the other creating more lift on that side. On a swept wing aircraft the lifting effect is even more pronounced. The extra lift on one side will cause it to roll.

And the right wing is better exposed to the airflow than the left wing which is partially in the shadow of the fuselage.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

sort: votes


text:
Where can one buy old 737 parts?
I'm trying to build a home cockpit for the PMDG 737-800. I'm looking for some landing gear position indicator lights but I don't know where to get them from. I've tried ebay and amazon, but it returned no promising results. The other alternative is of course to make them. Does anyone know where I could have something like this made?

Airliner parts have a tiny market outside of the airline and maintenance businesses (in the sense thst the general public generates no demand) which means that there probably aren't many places to shop for unairworthy (read: cheap) spares.  I would place "want to buy" ads on Barnstormers and maybe cruise the site from time to time clicking through relevent categories.  I did see one ad offering 737-200 parts but you'll have to contact the seller to see exactly what they have and how much they want.
You could certainly buy a new part but you will not like the price tag.  I imagine that an unairworthy spare for an -800 series will be hard to find compared to a -200, for example, because of the quantity of inoperative junk parts generated thus far by the respective fleets.
Good luck!

Thank you for that. I have now decided to make my own with a bit of 3D printing and laser cutting! Price tag will be sooo much cheaper!

There is a very large market for used airplane parts but you need to understand the types of parts you can buy. There are effectively 3 types of used airplane parts,

Red Tag: component is scrap or unusable
Yellow Tag: component is serviceable and airworthy
Green Tag: component is not airworthy but is repairable

when you are looking at aircraft parts most of what you will see is green and yellow tags as they are generally what is worth selling as they still have life. These parts are pricey as they still carry air worthy value. Most used aircraft parts come from air frames being parted out which often occurs at a bone yard. These same facilities also scrap some parts (red tag them). Since red tag parts carry little aviation value they can sometimes be had for cheap. You should contact the various large dismantlers with a list of what you are looking for and inform them that you would like red tag parts for a home simulator.
If you are just looking for instrumentation (which is in some regards generic to all aircraft) it may be worth a trip to your local airfield/maintenance facility to ask what they have lying around.
I would reach out to this guy and ask him where he got his stuff (I know some came from a bone yard).
I would read this, but take note of the fact that there is an active effort to physically destroy red tag parts so they don't make it back into circulation (unfortunately this has happened in the past due to fraud). This limits the public sale of these kinds of parts but there is not necessarily any law that I know of requiring the parts to be destroyed so you may just need to contact the right people at the right time.

Looking through the pages found on a search like this one, it looks like some old parts are out there as art pieces; perhaps one of these outfits could be helpful in locating either a part or a source for parts like those desired.

I find it interesting that Yellow tag items are serviceable and airworthy, while Green tag items need repair. Based on our conditioned response to traffic lights, I'd have thought the Green items were ready to go into an aircraft while the Yellow ones would require repair. (Yeah, of all the important things to jump out at me...)

Try cockpitsimparts.co.uk. I built my whole 737-800 from this site. Sells most things you will need at a very good price! Depends where you are I suppose. They are based in Cardiff UK.
Everything is made from plastic but looks pretty real!

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What material are High Pressure Compressors made of?
What material are GE engine High Pressure Compressor (spools) made of ? i.e. aluminum, titanium ?

Welcome to aviation.SE. This question is oddly specific, I think that an answer will be difficult to come by unless a GE engineering passes by and the answer is not a trade secret.

Thanks Airsick, titanium is my answer.  The 'document' -  "Materials for Gas Turbines – An Overview." ( Link > http://cdn.intechopen.com/pdfs-wm/22905.pdf ) was an excellent  resource.  The question was specific because of the British Airways 777 with GE engine GE-90-85 uncontained explosion at Las Vegas last month.  The NTSB has since said the compressor was the source.    - John King

@Pondlife I'm not sure that this question should be closed as a duplicate of that question, since this question was asked first.

@JonathanWalters The usual SE approach is to keep the question with the best answers, even if it's newer. It's the most pragmatic approach, I think.

Various alloys. In general , I believe even the same stage may have different alloys in various engines. As the air progresses through the compressor it heats and alloys are chosen accordingly. Over the years new alloy modifications are developed ; some more subtle changes are longitudinal grain orientation  and single crystal blades. I have done failure analysis on gas turbines, steam turbines and gas compressors and don't recall which blade went where. However, I never saw an aluminum blade ( or vane). GE , and others, have various 13% Cr steel blades they often used. As far as I know Ti and graphite are only used in the first stage fan. I only worked on older design engines, I expect Ti is used in earlier compression stages. The final stages of compression  are so hot that nickel alloys are used. I mean you need a very specific question to get a specific alloy.


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What is the fastest possible transatlantic flight today?
I'm working on a novel that, as part of the backstory, requires samples of a biological agent to be flown from Incirlik Air Base to labs across Europe, and then finally from the UK to the US.
In the context of the story, time is critical. Using any currently available aircraft (large or small, civil or military, US or allied etc.), what is the shortest flight time that could realistically be achieved?

Would an ICBM launch be acceptable ? It's most certainly the fastest way to get there. After this, the fastest way would be fighter jets if refueling is available, or bomber if not.

Under the circumstances an ICBM would probably not be a good idea. I think I'll go with fighters as suggested by you and KeithS.

One thing to consider is how far your labs are from the airport(s) that can handle your choice of plane.  For instance, you fly across the Atlantic to JFK in 3 hours, then spend 4-5 hours stuck in New York traffic.

How big is the sample though?  If it's a big container then you'd have to have at least a second seat to put it in.  Plus, if you're going for ultra plausible, I think you need to consider what aircraft are going to be available for the mission you are talking about.  Both in terms of geographical proximity, and how long it takes to prep the plane and how willing the air force will be to let you use it...

@JayCarr: Right, and don't forget the time it takes to get the tanker into its position. It needs to take off 3 hours before the F-22 to be ready, and this should be added to the mission time, too.

MiG-31 can cruise at Mach 2.35, but I'm not sure Russia would be eager to lend one.

@jamesqf - traffic?   Get the appropriate people remotely scared and of course the transatlantic flight would be met by a helicopter to shuttle the sample to the receiving lab

Space shuttle could do in 25 minutes but unfortunately in the opposite direction only.

The fastest air-breathing aerodynamic-lift craft for which a maximum speed is known to civilians, and that can support mid-air refueling for an unlimited range, is the SR-71 Blackbird at Mach 3.3. However, all known airframes of this model have been retired and mothballed as museum pieces, and the SR-71 runs on a special blend of jet fuel called JP-7 that is no longer in use by any production aircraft (the sole remaining use is for the X-51 scramjet testbed). So not only would you have to get the planes airworthy again (which would include repairing or replacing the wing spars, which were cut specifically to ensure the planes would never fly again), you'd need a batch of JP-7 at both ends and likely a tanker in the air full of it to use a Blackbird for this hop.
The fastest aircraft readily available and best suited for the job would likely be the F-22 Raptor. It can cruise at Mach 1.82 in clean configuration without afterburners (the samples could be contained in a "personal effects pod" carried in the internal bays), and has a one-way range of about 2,000 miles between refuelings.
The Eurofighter Typhoon can supercruise at Mach 1.5 and would be a good second choice or even a first choice, seeing as most of western Europe has these in service. You wouldn't have to get an F-22 over to Europe from an airbase in the States or the Middle East; there's a couple of RAF garrisons that operate the Typhoon about 30 minutes from Heathrow in Lincolnshire, and you only lose a few hundred miles an hour on the one-way trip.

Hmm, I had considered exactly the scenario you describe with an F-22 with drop tanks and internal travel pod. I wasn't sure how long it could sustain supercruise so I didn't know if there was perhaps a better tortoise option to the F-22's hare. Regular supercruise dashes and refuelling across the Atlantic sounds like the best solution, I'll just guesstimate how long it would take. Thanks.

Well, Mach 1.82 at cruising altitude of 30,000 feet is about 1235 mph. London's Heathrow to NY's JFK airport is 3741 miles, so if it supercruised the entire way at this speed it would make it in about 3 hours. With added time to get up and down, load and unload, plus it would need a midair refueling stop, you're probably talking about 4 hours realistically for a Raptor to get from Heathrow to JFK. That's about half the time it would take an airliner like a 777 or 787, but as I said this assumes a Raptor is just sitting on the tarmac at Heathrow; the Eurofighter could probably do it in 5 hours.

@qqjkztd - Highly implausible; "buddy refueling" is almost always reserved for dedicated elements of a larger package that head out alongside the combat units fully loaded with droptanks. Only way it makes any sense, and even then not really anymore; you lose a lot of fuel just lining everyone up to take a drink. The A-6 used to do this a lot for carrier-based strike packages, but that airframe's since been retired. Super Hornets can do it if they have to, but having a drogue-outfitted KC-10 (or the newer 767-based KC-46) on station from the Air Force is by far the preferable option.

@qqjkztd - Anyway, the Blackbird is so precisely engineered for speed that even swapping out the recon package of its predecessor the A-12 for a Phoenix missile and radar suite, resulting in the YF-12, proved problematic. Turning an SR-71 into a flying fuel tank including a refueling boom to refuel another Blackbird at supersonic speeds is just not going to sound like a realistic option.

Why were they so determined to keep the Blackbirds from ever flying again?

@Sean - First, it's US Federal law, not specifically for the Blackbird, but for "modern" combat aircraft (which AFAICT is about Vietnam-era). Certain designated combat airframes may not be transferred out of military ownership unless the aircraft has been rendered permanently unfit for flight and incapable of any combat capabilities it may have had. There are many ways they could have done that in the Blackbird's case; the primary way they did so was to make a "nick" in the wing spars with a cutting torch, so the aircraft can still support its empty weight on the ground but cannot fly.

If you prefer something more realistic and readily available, go with the fastest business jet, the Cessna Citation X, or if you need a little more range, the Gulfstream 650. The Cessna can cruise at Mach 0.935 over maybe 3000 miles (officially 3460 nm, but certainly a little less when flown at top speed), and the G650 has a range of 6000 nm at Mach 0.925. Thanks to @Lnafziger for helping with the facts. Bonus: Both are useable from a lot more airstrips than any airliner.
The flying time from London to New York will be less than 5 hours - still quite a bit faster than flying with an airline. By the way, the airliner record on this route is still held by the Vickers VC-10 with a tad above 5 hours. If you go from London to Boston, the Cessna will have just enough range, for all other routes I recommend the Gulfstream.
The bureaucratic procedure for making an F-22 available will give your protagonist ample time to cross the Atlantic dozens of times in a business jet. Add to this the stationing of a refueling tanker in the middle of the Atlantic (hint: That alone takes more time than what the F-22 needs to fly the full distance, so its realistic mission time will easily double).

Thanks for the link on Gulfstream 650, that is truly an amazing aircraft, with a range of 13,000 km non-stop...

Actually the cruise figure of Mach .935 for the Citation Ten is more of a marketing gimmick than anything else.  It’s true the airplane can achieve that speed but at the expense of clogging the crap out of the engines and very high fuel consumption.  Max cruise is at Mach .90.

A novel featuring a "biological agent" is almost certainly one where the people who can scramble fighters on a courier mission (and tankers to meet them) are going to be sufficiently worried to do so.  The premise of the question explicitly included military assets.

@dalearn: Thank you for letting me know. Cessna has discontinued the X because of poor sales, so they took the page down. I replaced the volatile corporate link with a more stable one.

@ChrisStratton Does being worried magically position tankers south of Greenland within the flight time of a supercruising F-22? Ah - I thought so!

Actually looking on a map at the great circle route from UAB to anywhere interesting in the US, even as far south as ATL and you'll see that tankers can fly from the UK, Iceland, and eastern US or Canada to cover three refuelings.  Distance the tankers fly is enough times shorter to make up for their slower speed.  And you could route the F22 even closer to their bases.  If you have to, land in the UK (or really handoff to a fresh pilot/plane) there's more time to prepare for the 2nd and 3rd refuelings.

Plus, it's a novel, having supporting characters just happen to be in position for some other actually happens in real life reason like a refueling a fighter group transiting the Atlantic is a pretty common literary coincidence.

The Internet Archive has several copies of the previously linked page on the Citation X from around the time of this answer here.

The shortest on record (and more or less possible) was done in an SR-71 Blackbird

Exactly 1 hour 54 minutes and 56.4 seconds later, they had set a new world speed record from New York to London England. The average speed was 1,807 statute mph over the 3,461 statute mile course

The Concorde set a commercial record and is the fastest practical plane you could get ahold of

on 7 February 1996 by British Airways' G-BOAD in 2 hours, 52 minutes, 59 seconds from takeoff to touchdown aided by a 175 mph (282 km/h) tailwind

The Areon company is working on a super sonic biz jet but it has not yet been released. For what its worth the SR-71 and Concorde are both no longer flying.
If we are talking about a science fiction setting there are faster "aircraft" out there like the X-15 rocket plane (the fastest maned in atmosphere vehicle ever built, however not air breathing) as well as many of the other planes from the X-Plane program. You could use any of the specs (or similar specs) to make up a plane of your choosing. For reference the fastest contraption we have dreamed up to date is the X-43 which clocks in at about 7000 MPH however it is un-maned.
For an added bonus effect (for realism) you can throw in something about needing to cool the biological agent in transit, in the plane since high speed planes (like the Blackbird) get very very hot at speed due to air friction.

Project Blackstar's speed (and very existence) is classified.
You can probably assume it can go Mach 6+ for sci-fi purposes.

Although the novel does contain one or two sci-fi elements I'm trying to keep the rest of the content grounded and realistic by comparison, so Project Blackstar might come across as a little too speculative under the circumstances. Thanks though.

This depends on how much warning you have beforehand.

Fighters like the F-22 and Typhoon can't do the flight without refueling. The shortest route is Halifax-Reykjavik, at 2000 miles this is just about doable by F-22. That means a stop, or you have to use mid-air refueling.
Mid-air refueling requires a tanker to arrive in the middle of the ocean to meet up with the fighter. If there's a few hours' notice, the tanker can set off at 900 km/h a few hours before the fighter with the samples takes off. If there's no notice, you can't do that.

If your story takes place in 2013, the fastest airliner back then was the Vickers VC-10. This could do the distance without refueling in 5 hours. It's now out of service. Current airliners are a bit slower.

Worst case, let's say you're flying from UAB to ATL to meet a helicopter to the CDC.  Look at the great circle route and it's obvious that a slower tanker out of either the UK or Iceland has plenty of time if promptly launched or re-purposed to get in position to meet partway.  Launch a second or third tanker out of the eastern US or Canada to intercept.

Getting moving now is often more valuable than moving fast.
I'm assuming this is a surprise, and the scientist runs into the ready room and say "I gotta go right now"  Otherwise, if the move is expected and nation-states have a couple days to pre-position airplanes, then it's just too easy - Russian Blackjacks are stationed only a couple hours away at Engels AFB, and it would be easy enough to get B-1-B's from the US.

So you are probably better off going Right Now with aircraft already on base, rather than wait for a fast aircraft to be marshaled from somewhere else.
And you're in luck.  That base has loads of KC-135's, which (with its 707 brother) are as fast as jetliners get, flying Mach 0.9. It'd fly at max speed and pay the fuel-economy penalty for doing so; fuel isn't a problem since it's a tanker.
And they're roughly the size of a 737/A320, so lots of airports can handle them. That matters because they can fly the cargo closer to the actual destination.
It defeats the purpose of flying supersonic if the supersonic aircraft can only land at an airport an hour further away by road.
The base also has F-15s good for 3000 miles one-way at best ferry speed. However, Berlin/London is 1500/2000 miles distance, so that's not a lot of extra fuel to be doing a whole lot of supersonic flight. Probably not worth the trouble, unless the F-15 was ready to go right now and the KC-135 was not.

Are you trying to argue there are runways that can handle a 707 but not a fighter?  Also you would be meeting a helicopter for the final transfer to the receiving lab.

@ChrisStratton No. I'm referring to that subset of strategic bombers that can fly faster than the 707 (and hence would be worth taking instead of the 707): Blackjack, B-1, maybe Tu-22M. Those are NOT optimized for short-field performance, since they're kept at few, well controlled bases where H-bombs can be secured. Could be true about fighters though; yes, some are made for guerilla-tier war where they're landing on farm roads and parking in barns; others are hyper-performance airport queens.  F-15 seems to be the latter.

The "White Swan" (Tupolev Tu-160) is the largest and heaviest Mach 2+ supersonic military aircraft ever built. It can reach 2,220 km/h but it is not clear what is the range at this speed. It can however cruise at 960 km/h for 12,300 km so could deliver the payload non stop, or alternatively make stops only as required by the plot.
Or let it run out of fuel while speeding at Mach 2 and it could pass the payload to F-22 arriving from another side, in some place where the both can land. F-22 could arrive a little earlier and be ready again at the time Tu-160 lands.

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When do we use IATA codes and when do we use ICAO codes?
I found on this website several questions speaking of IATA, ICAO and airports codes. The fact that two identification codes for the same airport exist means there are cases where you use one, and other cases where you use the other. I wonder what is the rule to know which code (IATA or ICAO) should be used.

Related: http://aviation.stackexchange.com/questions/8699/what-is-the-methodology-for-assigning-airport-codes/8701

You may book a ticket with a travel agency to DLP, which is the IATA code for Disneyland in Paris suburb. However there is no ICAO code associated, because the park has no aeronautical activity and is not even close to an airport.

It helps to know the objective of both ICAO and IATA to understand when which code is used.
ICAO (International Civil Aviation Organization) is a UN-body which focusses on international harmonization of civil aviation regulations.
ICAO codes are used for "official" purposes such as Air Traffic Control; E.g. flight plans use ICAO codes for airports and airline flight identification.
IATA (International Air Transport Association) is a trade association that focusses on making air traffic businesses profitable, safe, secure, reliable and efficient.
IATA codes are mainly used for ticketing. E.g. travel itineraries use IATA codes for airports and IATA flight numbers.

The IATA codes are 3-letter codes, since the number of commercial airports worldwide is enough that 3 letters are enough: DFW, LHR, NRT, etc.  The ICAO codes cover more airports, so they're 4 characters with the first (USA) or first two (most of the rest of the world) describing the region, and the last two unique to the airport there: DFW (IATA) = KDFW (ICAO) {K=USA}, LHR (IATA) = EGLL (ICAO) {since EG = Europe/Great Britain), and NRT = RJAA (RJ = East Asia/Japan). Your ticket might say DFW-LHR, but flightplan is filed KDFW-EGLL.

While pretty much all airports has an ICAO code, many smaller and non-serviced ones does not have an IATA. For example, look at small, unusual, high altitude airports. Basically, if they are marked on a flight map, it probably has an ICAO.With the obvious exception of countries/territories not conforming to int'l standards/rules.

I don't buy this "different codes for different purposes" explanation. Both assign codes to the same things so this just seems like pointless and confusing duplication.

@xjcl You don't have to buy it, but isn't it very common to have multiple codes for the same thing? this, 40885, 02102, Lintorf, 8RQM+VM; all these are codes for the city you are from according to your profile.

Of note, in the USA: Terminal-area air traffic controllers prefer the FAA Location ID rather than the ICAO code. For airports with an IATA code, the FAA LID is usually (though not always) the same as the IATA code.


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Do software simulations and wind-tunnel tests accurately predict Cd and Cl in real-world conditions?
Will the lift coefficient (Cl) and drag coefficient (Cd) I obtain by using software simulations or subsonic wind-tunnel tests be the same as on an actual aircraft while cruising? Or will there be some error?

There will be some error. I have plenty of experience with computational mechanics, and there is always an inherent error. The question is always whether the error is acceptable given the application, not whether there is an error at all. If unacceptable, you will need a more sophisticated model, either by a more sophisticated model or by obtaining experimental data.

Wind tunnel tests usually are "real-world conditions" for all practical purposes. That's why we have them and don't just use CFD for everything.

Also with wind-tunnel testing there will be differences coming from (possibly) different Reynolds numbers, different Mach numbers, different geometrical scaling, blockage effects, support effects etc. That's why there's still full-scale flight testing :)

There will be lots of errors due to the technics involved (method itself), and then there are the errors caused by configuration differences (aero configuration of the real flying thing (that are not modeled or are modeled with different geometries), and weight of the real thing, etc), and finally from the time the aircraft design is frozen till entry into service there are design changes which may influence the aero characteristics. That's why you fine tune the approximations during flight test phase with a prototype aircraft.

As mentioned, there are differences between CFD, wind tunnel and the real world. This is very evident in Formula 1, where the first practice session is used by the teams to do real world testing of aerodynamic pieces that they think will work based on the CFD & WT. Some teams spend an entire season struggling to figure out why what works in the lab doesn't work on the track.

It depends.
In computational fluid dynamics (CFD) there are limits for the calculation of turbulent flow. This means that large flow separation cannot be modeled precisely; especially its time-dependent characteristics are computationally much too complex to be modeled. Instead, statistical methods must be used.
For attached and far-field characteristics the methods are refined enough to yield very accurate results, so much that what had historically been studied in wind tunnel tests or only on the finished aircraft can now be calculated with confidence up to hypersonic speeds. Errors still exist - after all, if the geometric model does not include the manufacturing irregularities of the real aircraft, the results will not be in total agreement, but close enough for engineering purposes.
In wind tunnel testing you cannot expect the same results as long as the Reynolds number and Mach number are different from the real thing. This is adjusted with correction factors, and again the results become good enough for engineering purposes. Cryogenic wind tunnels even remove this restriction, but are very expensive to build and operate, because they run with pure nitrogen at very low temperature and several bars of pressure. Small errors remain, because all the details of the real aircraft cannot be modeled in a wind tunnel model and their ratio between dynamic pressure and stiffness cannot be adjusted for all flow conditions, so aeroelastic effects will remain different.
Windtunnel tests are still valuable for predicting forces in post-stall conditions and for calibrating new CFD codes. Most of the operational envelope can then be covered with CFD. The combination of both is enough to predict the force coefficients with confidence, and only aeroelastic effects and details of the stall behavior might turn out slightly differently than anticipated from tests.
As @TrebiaProject points out in the comments, CFD can help with the corrections. The influence of the sting the model is mounted on would be a primary example. CFD can help to understand its influence and correct for it. This makes CFD complementary, so both techniques will lead to better results if combined correctly.

In my experience, CFD only works great if you know the answer apriori. Once you've performed some wind tunnel test, you can always finagle CFD to match it; but if you're starting from scratch, CFD is almost always off (if not dead wrong). On the other hand, CFD is a great tool for generating data for design iterations or off-condition corrections.

@JZYL May I politely disagree? CFD is not trivial to use, but when you know what you are doing it saves a tremendous amount of time and the wind tunnel only gives a final confirmation of all that CFD work. Next, with CFD you can look into every corner of the flowfield in details that would be impossible in the wind tunnel. You use the wind tunnel to calibrate the calculation but what really enlightens you is the numerical result, not the tunnel data.

Windtunnels also have the challenge of corrections due to supports and attachments of the models. Those corrections can be predicted using CFD. WindTunnel and CFD are today complementary. Wind Tunnels provide high number of flying conditions meanwhile CFD provides high level of details for each solution. On top of what is described above.

@TrebiaProject.Thank you! Couldn't agree more.


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What is a typical compression ratio in light turboprop aircraft propellers?
Does anyone happen to know a rough estimate of a typical light turboprop aircraft propeller's compression ratio? Something with 6-12 seats like a Cessna Caravan or Piper Malibu. Doesn't need to be that accurate but something spinning around 1800-2000 rpm at sea level ISA?

Welcome to aviation.SE! I edited your question to (hopefully!) make it a little clearer. If I got it wrong then you can just roll back my changes or edit it again. Your title mentioned the Malibu, which is the PA-46, but you had tagged it PA-32, which is the Cherokee Six. I guess you intended the PA-46 because I think you're asking about light turboprops but again, don't hesitate to correct it if I'm wrong.

@Pondlife Turns out they really meant propeller compression ratio.

Mikey, please edit your question per your comment below.

By first principles, one simply expects the pressure difference to be thrust $T$ divided by disc area $\pi r^2$. Due to the fact that the propeller "sucks" ahead and "blows" behind itself, we'd expect to see about half of this pressure difference as a pressure drop ahead of the propeller and the other half as a pressure increase behind the propeller. The effective compression ratio $n$ in ambient pressure $p_0$ is then
$$ n=\frac{p_0+T/(2\pi r^2)}{p_0 - T/(2\pi r^2)}$$
To see what thrust is created for a given amount of power, you can use this excellent answer by Peter Kämpf.

We need an expression to relate thrust to power, as power is known, not thrust.

That equation is for static thrust. I guess the OP wants to get the power in flight, which is given in the linked answer. Thanks for your comment!

Couldn't get the equation in the proper form, so I removed it. You're right, it is for static thrust, but to me it is really unclear what the OP asks in terms of flight conditions.

Welcome.
https://en.wikipedia.org/wiki/Pratt_%26_Whitney_Canada_PT6
From Wikipedia, the compression ratio of the PT6 varies between 7 and 10.8, depending on the version. Please note the trend between compression ratio and engine power, due to this, I think the Caravan's engine (-114A) has a compression ratio of 7. The Malibu's engine (-42A) has a compression ratio of 8.
In off-design conditions (part-power, low RPM), the PT6 has a pre-swirl system that uses high pressure bleed air to change the direction of inlet air, thus widening the operational range of the engine. When compressed air is bled back to the inlet, the compression ratio obviously drops.

thanks for this but I was just wondering about the propellor not the engine. I just want to know the rough pressure behind the propellor. It can be from any light aircraft, even piston engined.

Engine compression ratio refers to the pressure ratio between the combustion chamber and inlet. For the pressure difference/ratio between the two sides of the propeller. I will refer you to momentum theory, https://en.wikipedia.org/wiki/Momentum_theory

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